Ground fault circuit interrupters (GFCI) and arc fault circuit interrupters (AFCI) are well known in the art. A GFCI is designed to detect a ground fault, which is an unintended conductive path between an ungrounded current carrying conductor and ground. The term “ground fault” generally includes both a “ground-fault” and a “grounded-neutral fault.” Ground-faults may be detected by sampling the current in a sense circuit on the secondary side of a transformer during a designated time interval and comparing the samples to a reference value. The sampling of the current is typically done by an ADC and involves conversion of the current from a continuous analog signal to digital data. Grounded-neutral faults may be detected by injecting current into the sense circuit during a separate time interval to produce a decaying sinusoidal signal in the sense circuit, then sampling the sinusoidal signal to determine the presence of a grounded-neutral fault.
An AFCI on the other hand, is designed to detect electrical arcing or arc faults. Arc faults are usually intermittent and do not generate sustained currents of sufficient magnitude to trip a conventional circuit breaker, so inputs such as band-pass filters, line current sensors, and voltage sensors must be sampled at regular intervals in order to detect an arc fault. Presently, combination arc fault circuit interrupters (CAFI) are available that can detect both parallel arcing (i.e., arcing between two conductors or between a conductor and ground) and series arcing (i.e., arcing across a break in a conductor, such as a damaged electrical cord).
Attempts to integrate CAFI and GFCI into a single, dual function CAFI/GFCI circuit breaker have met with mixed results. This is because, as a general rule, design strategies stress using a bare minimum number of components to achieve a desired function. Accordingly, most dual function circuit breakers use a microcontroller and one ADC to perform both the ground fault sampling and the arc fault sampling. However, while ground-fault sampling and grounded-neutral fault sampling may take place during separate time intervals, arc fault sampling can overlap with both ground-fault and grounded-neutral fault sampling. Such overlap can create conflicts requiring stringent timing constraints as well as other conflict avoidance measures in the microcontroller.
As an example, some dual function circuit breakers employ a complex scheme involving time slicing where the microcontroller allocates access to the ADC according to a precise schedule for the required samplings. In this scheme, the ADC is shared among multiple detection algorithms, each running in its own detection interval and each requiring different sequences of sensor signals to be sampled. Multiple priority-nested interrupts, timers, and state machines are needed to control program flow in such a scheme, which can render operation of the different detection algorithms less deterministic and more difficult to maintain.
Accordingly, what is needed is a way to minimize or eliminate timing and resource conflicts in a dual function CAFI/GFCI circuit breaker.